Gas turbine engines typically include one or more shafts supported on oil-lubricated bearings. These bearings are housed in bearing chambers, and there are seals between the chamber and the shaft to inhibit the leakage of lubricating oil. It is usually arranged that, under normal operating conditions, the pressure outside a bearing chamber is slightly higher than the pressure within it. This differential pressure ensures that there is a continuous counterflow of air inwards through the seals and oil leakage is avoided. However, under certain transient conditions the pressures may change so that the differential pressure is reversed. In these circumstances, oil will tend to pass through the seals and out of the bearing chamber.
Various types of sealing arrangement are known that attempt to prevent oil leakage. Labyrinth seals generally require a large and heavy buffer system to operate properly; some such systems also incorporate drains to dispose of any oil that does leak, adding further weight and complexity. Carbon seals can operate with a smaller counterflow of air, which may save weight in the buffer system, but they are still prone to allow oil to escape if the differential pressure is reversed.
A separate vent may be provided for the bearing chamber, to allow an outward flow of air, when required, other than through the bearing chamber seals. Examples of such devices are simple vents, with or without restrictors, and spring-loaded valves; but oil can still escape from the bearing chamber through these devices, and so they do not solve the fundamental problem.
Any oil that does leak out of a bearing chamber may contaminate the core air flow of the engine. When gas turbine engines are installed in aircraft, typically a proportion of the core air flow is taken to supply breathable air for the crew and passengers. The “cabin odour” arising out of this sort of contamination has long been recognised as undesirable. However, in recent years it has become increasingly clear that contaminated cabin air may also represent a serious health and safety hazard.